Journal
MATERIALS
Volume 8, Issue 8, Pages 5452-5466Publisher
MDPI
DOI: 10.3390/ma8085255
Keywords
heteroepitaxial systems and interfaces; rational design of nanoscale materials; structure-function relationship
Categories
Funding
- National Science Foundation [DMR-1409912, ECCS-0335765]
- Office of Naval Research [N00014-11-1-0665]
- National Science Foundation Materials Research Science and Engineering Center program [DMR-1120296]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02- 98CH10886]
- Division Of Materials Research [1409912] Funding Source: National Science Foundation
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Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.
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